Motorists wait in lanes of traffic heading into the United States from Mexico at the border crossing in San Ysidro, Calif., the busiest land border crossing in the US. An estimated 40 percent of the more than 10 million undocumented people in the US today came legally but stayed after their visa expired.

Mike Blake/REUTERS

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Washington — As the Senate’s “Gang of Eight” immigration reformers puts the final touches on legislation expected to be introduced this coming week, there’s a simple-sounding problem with a long-elusive solution that the group will have to finally nail down: how to figure out when and if the 150 million foreigners who come into the United States every year actually leave.

The policy of following who comes and who goes from America’s massive borders and hundreds of air and seaports, known in immigration wonk talk as “entry-exit” and formally as the US-VISIT program at the Department of Homeland Security, is one of the many issues the bipartisan group of senators vowed to tackle in their bill and is a particular priority for Sen. Marco Rubio (R) of Florida.

It’s not an academic issue: Somewhere in the neighborhood of 40 percent of the more than 10 million undocumented people in the US today came legally but stayed after their visa expired.

Although often overlooked next to the more emotionally charged issue of securing America’s southern border, the longstanding problem of figuring out how to make sure the nation’s legal immigration system is enforced is likewise vitally important – and going to be politically contentious.

“Without a real entry-exit tracking system, the rest of immigration law is irrelevant,” says Mark Krikorian, executive director for the Center for Immigration Studies, a group that advocates for lower immigration levels. “You can have Herman Cain’s electrified fences and any kind of other ridiculous border control measure, but if we’re letting people in legally and then not paying any attention to whether they leave, what difference does the border control make?”

Why is this so hard?

Beginning in the mid-1990s, Congress has asked for a system for tracking entry and exit from the US by non-US citizens a half-dozen times, upgrading its request from a “biographic” system of personal details to a “biometric” system of personal identifiers (chiefly fingerprints) after the 9/11 terror attacks.

While the US has achieved a nearly-universal level of biographic intake when foreigners come into the country, there’s little exit monitoring at the nation’s land borders and some monitoring at air and sea ports. And biometric screening is years away.

Today, all entering foreigners are subject to biographic and biometric screening. To determine when foreigners exit the country, that data is checked against airline flight manifests and, soon, Canadian data showing non-US citizens who cross the US-Canadian border.

But getting to the biometric standard, Department of Homeland Security (DHS) Secretary Janet Napolitano believes, would be ideal could take another half-decade or longer and cost as much as $10 billion, according to Republican aides on Capitol Hill who have studied the system.

In nearly two decades, then, Congress has asked for a viable entry-exit system on multiple occasions and gotten a half-loaf in return. (By the end of this year, Secretary Napolitano told a Senate hearing recently, DHS is aiming to provide country-by-country overstay rates.)

In that sense, it’s like a handful of other major border security requirements mandated by Congress that have gone unfulfilled for years.

That’s part of the overarching problem: with broad dysfunction in many parts of the immigration and border security system, it has been difficult to marshal the political will and financial resources to fix any one part without a broad overhaul.

“You have an automobile that has no tires, no wheels, no doors, no engine, and then, all right, great, you put two brand-new tires on it [and ask] ‘Why doesn’t it work?’” says Rep. Mario Diaz-Balart (R) of Florida, a key House immigration reform negotiator.

Privacy concerns play a role. While a massive database of all the foreign travelers to the US would hypothetically only be to make sure foreigners entering the country were following US immigration law, “the de facto effect is you have to build that same system” for Americans, says Chris Calabrese, legislative counsel for the ACLU.

Such a system not only risks misuse by the federal government in the future, Mr. Calabrese warns, but even small error rates marking US citizens as undocumented persons or visa overstayers could, among the millions of Americans coming and going from the country every year, place an enormous burden on US citizens.

There is also a host of practical problems that have stymied lawmakers along the way. First is a simple matter of infrastructure. On one hand, everything from airport terminals in Chicago to truck stops on the Mexican border are simply not built to accommodate the screening kiosks needed to evaluate such data.

On the other hand, the lack of exit screening at airports or when southbound at the US-Mexican border means potentially reengineering everything from border crossing points to airport jetways.

And because of that, imposing new technology burdens the flow of people and commerce across the nation’s busy ports. Republican congressional aides who witnessed a biometric scanning pilot project in Detroit noted that it greatly delayed passengers’ ability to disembark – and that possibility of slower service and longer transport times has everyone from airlines to trucking companies concerned about how to make a strong entry-exit program work without putting a serious crimp in their business.

“U.S. entry/exit regime is a government function – the government should own/implement it without imposing any unnecessary burden on airlines,” said a spokesman for Airlines for America, the trade association for major US airlines, in an e-mailed statement.

The problem of space and infrastructure also relates to the issue of cost.

Hill staffers who have talked to DHS estimate costs for still-undetermined technology and infrastructure changes could run between $3 billion and $10 billion over the next decade – and that it would take between three and five years just to get any biometric exit system up and running.

That issue of time and trust takes the problem full circle: Why should Congress obligate money to a mandate that has gone unheeded for so long?

“Who wants to give $10 billion to people who have been told for 15 years to develop [an entry-exit system] and they can’t even come in and give you a prototype?” says Rep. Bob Goodlatte (R) of Virginia, the chairman of the House Judiciary Committee, which has jurisdiction over immigration legislation.

Entry-exit and immigration reform

In that sense, entry-exit is very much like the rest of the border security debate: Can the border hawks and immigration advocates reach a compromise that both of them trust?

DHS’s stance on the matter is that not only do its statistics show the southern border more secure than ever before but that the current system is working to keep foreigners who would most likely threaten the nation’s security out of the country in the first place.

“We would like ultimately to be in a biometric environment, but that’s going to take some time and some real dollars,” Napolitano said at a recent breakfast sponsored by The Christian Science Monitor. “From a safety and security standpoint, what we’ve got and what we’re deploying really gets us about 99 percent there.”

With millions of overstayers in the country and few ways to measure who or where they are, border hawks blanch at such arguments that the nation is already doing a great job at border security.

“People need to accept the fact that this is part of border security – not just our physical borders and the perimeter of the United States but the interior as well,” says Rep. Lou Barletta (R) of Pennsylvania, who has raised sharp questions about the entry-exit program’s effectiveness in House Judiciary hearings.

And most important, Representative Barletta says, border security comes first – legalization for the undocumented in the US should come after.

That raises the hackles of advocates for immigrants, who see the pursuit of an ever-vanishing horizon of perfect border security as a straw man, a way for hard-liners to prevent anyone in the country illegally from becoming a citizen.

“The key thing to keep in mind is that the border is a perennial priority. We’re never going to be able to say border security is all done. It’s cynical to make 11 million green cards contingent on that goal,” says Angela Maria Kelley, an immigration expert at the liberal Center for American Progress.

To compromise, Representative Goodlatte of Virginia suggested that a mandatory employment verification system put in place immediately could help dissuade visa overstayers until the full biometric system is put in place later this decade.

That still puts a final answer to a nearly two-decade old question off for another handful of years.